1. Field of the Invention
The invention relates to a drive unit for a synchronous ion shield mass separator having a reference oscillator, a digital direct synthesizer, a low-pass filter and a comparator, wherein the synchronous ion shield mass separator has a comb-shaped separation electrode, the reference oscillator provides the direct digital synthesizer with a reference frequency, the output signal generated by the direct digital synthesizer is filtered by the low-pass filter and the output signal of the low-pass filter is processed by the comparator. The invention further relates to a method for driving a synchronous ion shield mass separator, wherein the synchronous ion shield mass separator has a comb-shaped separation electrode.
2. Description of Related Art
Mass separators of this type aid, in mass spectrometers, in separating charged particles—ions—according to mass or according to their mass/charge ratio and are thus also called analyzers. The mass separator makes up a substantial portion of the entire spatial requirements of the mass spectrometer. In the scope of miniaturizing mass spectrometers, it is thus of particular importance to develop a particularly small, yet still high-performance mass separator that further separates ions with extreme precision. Such a mass separator is described, for example, in the article “Mass spectra measured by a fully integrated MEMS mass spectrometer” by J.-P. Hauschild et al., International Journal of Mass Spectrometry, Elsevier, March 2007 and is called a synchronous ion shield mass separator there.
A synchronous ion shield mass separator consists essentially of a comb-shaped separation electrode. This comb-shaped separation electrode has a plurality of teeth, which are arranged next to one another at short distances on the comb ridge so that a small gap remains between the teeth of the separation electrode and the comb ridge. Often, the comb ridge also has small protrusions that are located opposite the teeth. The ions to be analyzed are charged with energy by an electrical field—as a function of their charge—and accelerated—as a function of their mass. After passing through the electrical field, the ions have an identical direction of movement. The electrical intensity of the field, on the one hand, and the mass and the charge of the ions, on the other hand, determine the speed of the ions after passing through the potential difference.
From one end of the gap, which is the entrance of the mass separator, the accelerated ions are placed parallel to the comb ridge in the mass separator. The mass separator is normally evacuated as far as possible, so that the ions can easily move along the gap. The requirements for the evacuation of a miniature mass separator are not as strict as that of a non-miniature mass separator, since the ions in a miniature mass separator only have to travel a very small distance and thus the possibility of impact with residual gas atoms or molecules is minimized.
By creating a voltage between one tooth and the comb ridge of the comb-shaped separation electrode, an electrical field is generated that diverts ions moving through the gap from their original direction of movement, so that they collide with the comb-shaped separation electrode and do not reach the other end of the gap, the exit of the mass separator. Depending on the charge of the ion and the direction of the electrical field, diverted ions collide either with the teeth or the comb ridge of the separation electrode. These diverted ions are no longer available for further analysis should, for example, the mass separator be inserted in a mass spectrometer.
It is known from the prior art to apply a voltage between every other tooth and the comb ridge and to apply no voltage between the teeth located between them and the comb ridge. In this way, a simple pattern of alternating applied voltage and non-applied voltage results along the teeth, called signal sequence in the following. A simplified representation of such a signal sequence occurs here with zeros and ones, wherein a one represents the presence of an electrical potential difference and a zero represents the absence of an electrical potential difference. The signal sequence described above of alternating applied voltage and non-applied voltage thus corresponds to a signal sequence of alternating zeros and ones. In a comb-shaped separation electrode having 10 teeth, the result of strictly alternating presence and absence of a potential difference is: